The problem of determining the spatial position of objects is an ancient one. Perhaps the simplest and oldest known solution is to pace off a distance to a visible object by walking toward it along a straight path. More accurate and recent techniques include triangulating the location of a hidden object based on estimated distances or azimuthal angles to the object.
Measurement of azimuthal angle to a given object tends to be less accurate than measurement of distance to that object. Extremely precise instruments have been developed for distance measurement. For example, an optical instrument disclosed in U.S. Pat. No. 5,430,537 to Liessner et al. purports to have accuracy around the 1–10 micron resolution of light wavelengths. This instrument is based on phase changes between a light beam sent to a passive reflector and another light beam returned from the reflector.
Less precise instruments for phase-based distance measurement can provide benefits in particular applications. For example, R. S. Trenam, “Automatic Animal Tracking on a Limited Budget,” in The Collection and Processing of Field Data (1967) (pp. 273–82), discloses tracking of sheep to 20-yard accuracy using RF phase measurements.
In any system relying on phase differences between forward and return signals, frequency stability of the signals is critical to maintaining accuracy of distance measurement. Slight frequency deviations in the forward and return signals can cause significant phase deviations, especially when the distance to be measured includes a large number of wavelengths. Such phase deviations interfere with those expected from changes in distance and can significantly degrade accuracy.